The present invention relates in general to a mechanism for dispensing a mono-filament strand at a high speed and pertains, more particularly, to a mechanism for dispensing a fiber optic cable between two objects moving relative to each other or one object moving relative to another. The mechanism of this invention provides a pay out device for dispensing fiber optic cable for communication between a weapon launched from an aircraft during flight and the aircraft.
With a conventional launch platform to launched weapon communications systems radio signals are transmitted through the air to maintain a desired communication between an aircraft and a weapon during flight of the weapon from the aircraft to the target.
Numerous strategic and tactical difficulties with these systems were likely incentives that spurred the development of wire guided weapons systems. It was recognized that radio frequency data links have numerous drawbacks associated with high-frequency electromagnetic (i.e., radio) transmissions through the air.
These drawbacks include terrain blocking high carrier frequencies required for high-bandwidth video transmissions requiring that a pilot of a launch aircraft must keep the launched weapon in sight until it hits an intended target. Plainly, if the pilot of the launch vehicle can see the weapon as it hits the target, then an air defense facility can see the launch aircraft, thereby making the launch aircraft vulnerable to anti-aircraft defenses.
Furthermore, radio frequency transmissions directed from an aircraft toward a weapon and the target are readily detected at the target defenses, thereby providing an unintended alert of the imminent attack. It is also possible to disrupt radio frequency transmissions in both directions, once detected, by jamming and thereby rend&ring the weapon ineffective. The development of wire guided weapon systems was a response to these and other drawbacks associated with radio frequency transmission data links.
The drawbacks of the high frequency radio frequency transmissions are typically inherent in the system and, therefore, generally inescapable and result in major tactical limitations. The detection and jamming drawbacks may be countered by use of sophisticated, expensive, and heavy frequency hopping, low-probability-of-intercept transmission techniques.
With a conventional wire guided weapon system a pay out mechanism for a thin wire is generally necessary to provide communications between the launching aircraft and a launched weapon. It is known to use the thin wire for communicating guidance and control signals.
The wire guided weapon systems have their own set of drawbacks stemming at least in part from the medium of signal transmission, that is the use of electrical transmissions through conductive wire. These systems are susceptible to jamming by enemy jamming signal sources.
The wire guided systems require a complete circuit path and therefore two wires. Both wires must pay out simultaneously. The use of two wires doubles the weight and complexity of the system with a corresponding probability of failure. The wire guided systems are limited in the use since operational flight paths are required that avoid entanglement of the wires.
It is desired that the wires are light and compact and therefore essentially unshielded and they tend to produce large electromagnetic fields. The field generation requires a large amount of power, thereby limiting the bandwidth. The result is a control system that can transmit control signals only from the launch point to the weapon. Thus, two-way transmissions at even low data transmission rates are not possible. This further means that high data transmission rate systems, such as video, are completely out of the question with wire guided systems.
The electrical potential or voltage developed between the two wires at the end associated with the transmission source must remain in part at the receiving end for the system to function. However, moisture and other conductive elements in the air, on the ground, or in the water with which the wires come into contact can partially or totally short-circuit the electrical path. This prevents any signal from reaching the receiver. The result is an inoperable data link.
The wire pairs form, in effect, an antenna. This antenna radiates the control signals being carried as well as capturing signals being broadcast around it. The wire data link is now detectable and jammable. It is known to counter enemy jamming signals of radio and direct wire communication links with equipment and techniques which are both expensive and sophisticated.
These known techniques typically encumber a weapon system with drawbacks such as additional costs, weight and/or size of the system, and overall operational complexity of the system. All of these factors are known to contribute to a lack of weapon system reliability.
The wire guided system drawbacks are inherent in the system and cannot be eliminated. Other transmission techniques that are available cannot be used because of the bandwidth limitations of a wire guided system.
These factors undoubtedly spurred the further development of wire guided communications systems in the direction of fiber optic communications systems. Fiber optic communications systems and data links of the type pertinent to the present invention offer a number of advantages.
They have lower energy requirements. The actual power transmitted through the fiber waveguide data link will be on the order of a milliwatt. The total electrical power consumption is on the order of twenty-five watts.
There are no detectable energy emissions. The energy radiated from the optical fiber data link is extremely low density. Thus, the probability of detection from energy radiation is insignificant.
Fiber optic data links resist jamming. An external signal (the jamming signal) cannot be directly coupled into a conventional functioning single mode fiber optic data link. The only way to jam a fiber optic link is to bathe a portion of the link in such an intense light at particular frequencies that the fiber begins to fluoresce.
However, even if the would be jammer detected the hair-fine, transparent fiber by means other than emissions, the density of the jamming energy which must be transmitted to intercept the invisible fiber would be enormous, for example on the order of several watts per square centimeter at the fiber and in the fluorescence-inducing bandwidth.
Fiber optics allow non-line-of-sight operation. Since the signals are guided down the center of an optic fiber, they will follow a curved fiber. Thus, the weapon can be launched from behind the cover of terrain and the launch platform can stay out of sight even while travelling away from the target for the entire duration of the weapon's flight.
An optical fiber waveguide data link functions in the high bandwidth range. Thus, the optical fiber waveguide data link will handle black and white or color video bandwidth from the weapon to an aircraft over extended ranges. The fiber optic data link allows receipt of video signals and simultaneous transmission of control signals in the opposite direction.
Yet, with all of the positive aspects of the fiber optic system, drawbacks still emerged. These drawbacks are associated with using fiber optic links for communications purposes and particularly where such links are required to be established and maintained between relatively moving objects, such as, a launch platform and a launched weapon.
Fiber optic links are known for use between missiles or bombs and launch vehicles which are fixed (e.g., truck mounted launcher) and mobile (e.g., attack aircraft). Both the fixed and mobile applications guide the weapon in flight. However, it is recognized that difficult problems and complexities are associated with fiber optic communications links between two high velocity airborne vehicles moving relative to each other in an hostile military operations environment.
It is recognized that a fiber optic link between two vehicles moving at a high velocity relative to each other is susceptible to breakage, entanglement, and other operational and environmental stresses. These drawbacks can adversely effect the physical integrity, function, and performance of the fiber optic communications system. A significant drawback exists in a potential for the fiber optic cable to become entangled or break during pay out from conventional free-helix fiber optic payout devices.
Conventional free-helix pay out systems have numerous problems associated with the three stages of their existence. First, protecting the delicate fiber from moisture, dust, and physical damage prior to weapon release. Second, initiating pay out by releasing the fiber. Third, permitting fiber pay out after release without breaking it.
All of the foregoing drawbacks stem from a free-helix pay out in which the fiber leaves the launch vehicle traveling in a relative direction opposite the flight path of the launch vehicle. Furthermore, the fiber is on a cylindrical surface with the same direction as the bobbin, which intercepts the bobbin's axis at the peel point.
During fiber pay out the fiber motion and path is determined by fiber tension and aerodynamic forces. If, for any reason, the bobbin or any other interfering surface of the vehicle intercepts the fiber path, then fiber damage and failure of the data link is likely.
As a result, the bobbin must be at the rear end of the launch vehicle, facing directly aft and in line with the current flight path. If the launch vehicle experiences any significant angle of attack in the pitch or yaw planes, then the fiber risks probable breakage.
Prior to weapon release and pay out a large circular hole through which a free fiber helix exits must be covered. The exit hole must be covered in such a way as to protect the optical fiber data link and the remainder of the vehicle interior from all external sources of damage.
In general, the larger the circular hole, the more difficult it is to effectively cover. The entire perimeter of the hole must stay environmentally sealed over a wide range of conditions. A six inch hole, for example has almost nineteen inches of perimeter over which a suitable seal must be maintained.
The cover referred to above must have the enviromental seal instantly and reliably removed from the hole at pay out initiation. Cover removal must occur in such a way so as to pose no threat of damage to the fiber, the weapon, the launch aircraft, or other aircraft flying in formation with the launch aircraft. The requirement of rapid release in only a few milliseconds and the transition from a tight environmental seal to a complete release is a significant reliability problem.
The hole referred to above creates a point at which the aircraft can be observed by hostile forces. Whether before, during, or after flight, the large hole in the launch aircraft through which the fiber exits will likely have different dielectric behavior than that of the surrounding surface of the aircraft. This observable discontinuity will result in a reflection at radar frequencies which can be unacceptable for aircraft incorporating low-observable technology.
Another drawback associated with fiber optic pay out systems is the problem of housing the cable in order to allow simultaneous pay out at a high rate of speed between a moving launch aircraft and a weapon released from the aircraft toward a distant target.
The optical fiber is normally stored and transported on spools or bobbins having a generally cylindrical shape or a tapered cone-like shape. The optical fibers are typically wound in a tight, closely packed helix about the outside diameter of the bobbin. When the optic fiber is dispensed, or paid out from the spool on the bobbin at high speed, it is known to pull the optic fiber off the bobbin in a direction generally parallel to a longitudinal axis of the bobbin and toward a small or truncated end of the cone.
It is known and frequently observed that if the optic fiber is wound on the bobbin in a helix pattern and then pulled from a point distant from the small end of the bobbin and along the longitudinal axis, then the optic fiber leaves the bobbin in the form of a helix. The helix of optic fiber has a helix diameter that gradually decays from a diameter approximately equal to the bobbin diameter to a diameter of essentially zero. The decay typically requires the pay out of optic fiber equal in length to many hundreds of bobbin diameters.
The form of the substantially unrestrained or free-helix and the helix decay rate are functions of the geometry of the bobbin and the fiber. Other factors include the bulk material properties (e.g., fiber density and stiffness are properties of primary concern), the coefficient of friction of the optic fiber on itself, and the drag characteristics of the fiber in the respective medium (e.g., air) through which it is paid out.
In many applications the fiber must be constrained to a helix diameter which is relatively small in comparison with the bobbin diameter, through a pay out length shorter than that typically required for the natural decay of the free-helix. This is normally accomplished by guiding the fiber through a conventional rigid guide ring having a desired inside diameter and oriented in a plane perpendicular to the axis of the bobbin and also concentric with this axis.
A serious drawback of these pay out systems is that the physics of the pay out dictate that the conventional rigid guide ring constrains the helix with a resulting tendency of the optic fiber helix to "balloon" resulting in the swelling of the helix diameter before the optic fiber has passed through the conventional guide ring. The swelling takes the shape of a smooth curve outward from the point where the optic fiber leaves the bobbin and before curving down to the diameter of the constraining ring.
The shape of the aforementioned curve is determined by the above indicated factors, the diameter of the conventional guide ring, and the distance from the conventional guide ring to the point where the optic fiber leaves the bobbin. Increasing the pay out velocity increases the maximum diameter of the "balloon". This increases the tensile and bending loads placed on the optic fiber as it passes through the conventional guide ring.
Yet other drawbacks to conventional fiber optic pay out systems include two practical velocity limitations in the actual application and use of conventional pay out systems. One limitation results from the tendency of the optic fiber to balloon as the velocity increases resulting in potential interference between the optic fiber and adjacent, fixed components of any associated fiber optic pay out mechanism.
Another limitation is the actual loading on the fiber itself of static and dynamic stresses as it passes through the conventional guide ring. These loads may eventually reach the strength limit of the optic fiber at a particular velocity resulting in fiber failure. The bending loads on the optic fiber are a particular concern for the optical fibers of a weapon system. Optical fibers are typically stiff, brittle monofilaments.
It will be recognized by those skilled in the art that the limitations related to optical fibers are applicable to monofilament strands in general as they are paid out through a constraint such as a guide ring. It will be understood from the objects and description of the present invention that this invention is readily applicable to solving equivalent problems in the bobbin, textile, and wound fiber art.
It is believed that the similar problems encountered in the textile industry with conventional textile fiber pay out mechanisms, the pay out mechanisms used in the manufacture of filament wound structures in general may use a mechanism similar to that set forth in the following specification and claims. However, for the purposes of clarity and precision, the described embodiment will be limited to a launch platform (e.g., aircraft) and weapon combination.
Two additional stress problems related to the free-helix pay out of mono-filament strands can occur if the natural helix decays rapidly enough such that the bobbin structure prevents the optic fiber from following that helix. These conditions can occur when the mono-filament strand pay out is from the end of the bobbin furthest from the direction of pull.
One stress problem has been observed to occur when the taper angle of the conical bobbin is less than the decay angle of the pay out helix. The optic fiber tends to wrap tightly around the bobbin, drag along the bobbin, and pull sharply in the direction of the longitudinal axis at the end of the bobbin.
Another problem occurs as a result of conditions in which the axis of the bobbin is not aligned with the pay out direction and the bobbin does not include the guide surfaces set forth below. In this condition the optic fiber is trying to pull its way through the bobbin along the bobbin surface farthest from the pay out direction. The fiber optic is forced to drag across the corner of the bobbin.